Week 5 Bollough Lecture 8

Question 1

What does the sequence analysis say about aquaporins?

Answer 1

Each monomer has a pseudo 2fold symmetry, i.e. an inverted topology repeat;
a) inverted topology repeats are common amongst membrane transport proteins- they indicate an ancient gene duplication event

Question 2

How do aquaporins work?

Answer 2

Include:

a) The function of the half helices;
i) Negative part is near the outside of the pore, and it attracts water molecules
ii) Positive parts meet in the middle
b) Histidine is used to narrow the passage because of the ring
c) Arginine used for repealing positive ions (electrostatic repulsion)?
d) Asparagine is used to select water and/or stop protons from moving inside

Question 3

Why are protons not co-transported through the protein?

Answer 3

If the water channel in aquaporin contained an uninterrupted chain of water molecules then protons would also be transported via a “proton conducting wire”
a) Two crucial conserved asparagine residues are H-bond donors to water in the channel, breaking the “wire”

Question 4

What is the orientation of water molecules inside the membrane like?

Answer 4

as each water molecule reaches the centre, it forms H- bonds to the two asparagine residues

a) the dipole orientation flips
b) so, every water molecule is H- bonded in the channel